A deactivated mailbox is usually one of two things:
Billing. If a payment failed, your accounts are paused until billing is updated. Update your payment method from your billing page to reactivate.
Sending limit. Mailboxes are automatically deactivated if they exceed 100 emails per day per mailbox. To stay safe, run warmup at 80 emails/day per mailbox, and after the 14-day warmup period cap campaign sending (including follow-ups) at 15 emails/day per mailbox — about 95/day total, which leaves room under the 100/day cap.
For the full breakdown of safe warmup and campaign volumes, see What's the daily email limit per email account?
Contact our support and we'll reactivate the affected mailboxes. Alternatively, if the mailbox is marked as "is_active false," you can navigate to the Email Accounts page in your dashboard and toggle it back on to reactivate it. Note that this applies only to mailboxes that have been manually turned off and not those deactivated due to billing or sending limit issues. Note that deactivated is not the same as deleted — a deactivated mailbox still exists and can be turned back on; see Deleting, replacing, reducing, or cancelling mailboxes and domains if you're trying to work out which one applies to you. A mailbox marked as "is_active false" is still provisioned but has been turned off in the account settings. It will appear inactive in the dashboard and cannot send emails until reactivated.